BARBARA McCulkin was murdered because she was "blackmailing” one of her accused killers who feared he would go to jail for 20 years over a nightclub fire, a court heard on Monday.

Crown witness Paul Dubois made the claim while giving evidence before a Brisbane Supreme Court jury.

His brother, 69-year-old brother Garry Reginald 'Shorty' Dubois, is on trial for the murder of Mrs McCulkin and the rape and murder of her daughters Vicki, 13, and Leanne, 11.

Paul Dubois said his brother told him co-accused Vincent O'Dempsey killed Mrs McCulkin because she knew something that would get Mr O'Dempsey "20 years” in jail.

Paul Dubois also said Mr O'Dempsey said Vicki and Leanne were not supposed to be with their mother when she was allegedly murdered.

The Highgate Hill mother and her daughters disappeared from their home on January 16, 1974.

Garry Dubois, from Torbanlea, has pleaded not guilty to one charge of deprivation of liberty, two of rape and three charges of murder.

Mr O'Dempsey, a 78-year-old Warwick man, is scheduled to face trial next year.

Paul Dubois said he spoke to his brother not long after seeing a show on television that implicated Garry Dubois and Mr O'Dempsey in the McCulkins' disappearance.

"He (Garry) said Vince O'Dempsey did it,” Paul Dubois said.

"He said she (Mrs McCulkin) had information on O'Dempsey that he felt would have got him 20 years and this was how he dealt with it.

"He (O'Dempsey) said she was blackmailing him.

"Vince said they (Vicki and Leanne) weren't supposed to be there.”

Paul Dubois admitted under questioning that he had memory problems.

However, he said he recalled Garry telling him "he wasn't there and had nothing to do with it”.

The court previously heard the McCulkins may have had some knowledge of the 1973 Whiskey Au Go Go nightclub blaze that killed 15 people in 1973.

Crown prosecutor David Meredith told the court that Peter Hall, Garry Dubois, Mr O'Dempsey and two other men were behind the torching of the Torino nightclub about 10 days before the Whiskey Au Go Go went up in flames.

The court has heard that the accused feared their involvement in the Torino blaze could see them linked to the Whiskey fire.